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Effects in the Rat of Environmental Temperature, Diet Dilution, and Treadmill Running on Liver and Adipose Enzymes and Metabolism of ¹⁴C-Glucose: A Multiple Regression Analysis¹

Nutrition Laboratory, Department of Dairy and Animal Science, The Pennsylvania State University, University Park, Pennsylvania 16802

The effects of two environmental temperatures (T; 16° and 31°), five diet dilutions (D; 0%, 12.5%, 25%, 37.5% and 50%), and five daily treadmill running periods (E; 10 minutes, 40 minutes, 70 minutes, 100 minutes, and 130 minutes) upon enzyme activities of liver and adipose tissue of male rats were observed. Liver enzymes studied were glucose-6-phosphatase (G6Pase), 6-P-gluconate dehydrogenase (6PGD), glucose-6-phosphate dehydrogenase (G6PD), fructose diphosphatase (FDPase), NADP-isocitrate dehydrogenase (ICDH), and malic enzyme (ME). Adipose tissue (epididymal fat) enzymes (6PGD, G6PD, and ME) were studied as well as the in vitro incorporation of the ¹⁴C of [U-¹⁴C]glucose into liberated ¹⁴CO₂ and into the triglycerides, free fatty acids, and total lipids by adipose tissue slices. Equations describing regression surfaces for these responses (expressed as units/100 g body weight) could contain significant linear coefficients of the independent variables (T, D, and E), their first order interactions, and quadratic coefficients for D and E. Significant regression coefficients for activities of liver enzymes associated with increased lipogenesis (6PGD, G6PD, and ME) produced response surfaces with conformations generally concave downward. All enzymes possessed positive and negative linear and quadratic coefficients for D which caused response surfaces to be concave downward with respect to that variable. Also, 6PGD and G6PD (positive linear and negative quadratic coefficients for E) exhibited response surfaces concave downward with respect to E. Additionally, 6PGD showed greater activity at 31° than at 16° while G6PD showed no effect of temperature on activity. Liver ICDH, probably important in supplying reducing equivalents for fatty acid synthesis, evidenced response surfaces almost identical to those for 6PGD. Significant regression coefficients for activity of liver enzymes associated with increased gluconeogenesis (FDPase and G6Pase) produced for FDPase a response surface concave downward with respect to both D and E with greater values at 31° than at 16°; but for G6Pase non-concave surfaces with lesser values at 31° than at 16°. Significant regression coefficients for activities of adipose enzymes associated with increased lipogenesis produced for 6PGD a response surface concave upward due to negative linear and positive quadratic coefficients for both D and E. For G6PD and ME regression surfaces were concave upward with respect to E, but these were modified by positive and negative linear coefficients, respectively, for D. Significant regression coefficients for incorporation of the ¹⁴C of glucose into triglycerides and free fatty acids of adipose tissue slices and their production of ¹⁴CO₂ yielded response surfaces concave upward with respect to E (negative linear and positive quadratic coefficients). In addition, the surface for free fatty acids was concave upward with respect to D. The ¹⁴CO₂ production was greater at 16° than at 31°. No significant coefficients were observed for ¹⁴C incorporation into the adipose lipids.

KEY WORDS: • environmental temperature • energy • exercise • liver enzymes • adipose tissue

¹ Authorized for publication on May 23, 1975 as paper no. 4871 in the journal series of the Pennsylvania Agricultural Experiment Station.

Manuscript received 9 June 1975.